This invention relates to electrical resistance heaters, and more particularly to such heaters in which the heating element is formed of electrical resistance heating wire, of nichrome or the like, and even more particularly in which the heating element is formed in a helical coil and in which the heating element is supported on a frame or the like by means of ceramic-like insulators. Such heating elements find a wide range of applications, such as in electric clothes dryers, resistance heating systems for residential and in commercial heating and ventilating systems.
Heretofore, electrical resistance heating elements, such as described above, were mounted on a metal frame and were carried by ceramic electrical insulators thereby to electrically isolate the heating element from the frame. As shown in U.S. Pat. No. 3,697,727, these insulators were typically hollow collars or bushings through which the coiled electric resistance heating element was inserted. However, it has been found that these ring-like insulator collars were not entirely desirable because air flow through the collars was somewhat limited and thus increased significantly the surface temperature of the electric resistance heating element within the collar several hundred degrees Fahrenheit above the temperature of the heating element exposed to air flow thereover. This increased temperature of the heating element within the collars deleteriously affected service life of the heating element because of increased rates of oxidation of the heating element wire. Further, the insulator bushings did not securely fasten or anchor the heating coils and, during repeated firing and unfiring of the heating element, the coils would slide on the collars and abrade the heating element, weakening it and leading to reduced service life of the heating coil.
Additionally, as is typically illustrated in the above-noted U.S. Pat. No. 3,697,727, the heating element was typically of helical coil construction from one end to the other with the heating coil being formed in a generally serpentine arrangement with the parallel portions of the heating element constituting runs and with the portions of the heating element interconnecting adjacent runs being referred to as turnarounds. Typically, such a continuous, coiled heating element is installed in an air duct or the like through which air may be forceably circulated thereby to transfer heat from the heating element to the air flowing therethrough. This compact, serpentine design of the heating element is desirable because it maximizes circulation of the air over the heating element and thus enhances heat transfer to the air. However, the lengths of the heating element runs which are freely supported between the insulators tend to lose strength when heated or fired and, over time, the weight of the coiled helical heating element hanging between the insulators could cause the heating elements to sag. This sagging is particularly undesirable because if the heating element sags beyond a certain degree because the heating element may break or may contact the sidewalls of the duct within which it is installed or the frame carrying the heating element thus shorting out the heating element. Therefore, there has been a longstanding need to support the heating element so as to eliminate or lessen the tendency of the unsupported lengths of a colied heating element to sag.
Further, it has heretofore been conventional to form the entire length of the heating element of helical coiled construction including not only the runs of the heating element supported between the insulators, but also the turnaround portions interconnecting one run of the heating element with another. It will be appreciated that as the helical heating element is turned on itself in the turnaround, the portion of the helical coils on the inside radius of the turnaround come much closer together. This has a particularly deleterious effect on the heating element because the closer spacing of the coils significantly changes the reradiation coefficients of the coils relative to one another thereby significantly increasing the temperature of the more closely spaced coil portions and, as heretofore pointed out, increased temperature levels of the coils results in reduced service life due to increased oxidation of the coils.
Still further, the requirement of having to use solid circular insulator collars required more time to assemble the heating element because the insulator collars had to be threaded onto the continuous length of the heating element.
Reference made to such U.S. Pat. Nos. as 1,844,678, 2,921,172, 3,016,441, 3,358,074, 3,641,312, 3,770,939, 3,846,619, 3,890,487 and U.S. Pat. No. Des. 262,285 for prior art references in the same general field as the present invention.